Making Waves: Dr. Silver Studies Seafloor Slumping.

It’s a series of events made familiar by big-budget Hollywood movies. Sitting on the tranquil shore, staring out at the sea, a frown wrinkles the brow of our protagonist.  Panning back to the expansive ocean, something is amiss - the horizon has begun creeping closer to the shore.  The seconds tick by, and large and grey, a wall of water advances.  Slow motion turns into frantic terror as people begin to point and scream and run in every direction.  Tsunamis provide excitement and drama for our summer blockbusters.

However, for those in the Pacific Basin in the hours following the February 27 magnitude-8.8 earthquake off the Chilean coast, the fear of tsunamis made the jump from the big screen to reality.

The U.S. National Weather Service's Pacific Tsunami Warning Center issued a tsunami warning throughout a huge swath of the Pacific region, including Hawaii, California, Oregon and Washington. People in coastal towns up and down the West Coast watched both the news and the shoreline for the sight so vivid in their mind’s eye thanks to Tinsel Town.  Fortunately for them, immense tsunami waves stayed the stuff of Hollywood CGI.

Not so for the people of Papua New Guinea.  Although not affected by the 2010 quake, this southwestern Pacific island nation is no stranger to the horror inflicted by tsunamis.  Nor are they strangers to the probing eye of research scientists.  Situated in the Bismarck Volcanic Arc, Papua New Guinea has been a popular destination for those studying tsunami generation.  Among them, Eli Silver, a professor of earth and planetary sciences at UC Santa Cruz.

It has long been understood that earthquakes can cause tsunamis.  But Silver and colleagues suspected there had to be more to the story.  Take for example the 7.1-magnitude earthquake off Papua New Guinea in 1998; it was not large enough to generate the tsunami that left thousands dead.  If not the quaking seafloor, then what caused the giant wave to rush the shore?

”Anything that moves the seafloor up or down will cause a tsunami,” said Silver, a 1969 graduate of Scripps.  That seems like a simple enough statement, but its validity pushed open wide a door that had only been cracked before.  Thinking beyond the Richter scale, scientists began positing what else could generate these perilous super waves.  “Earthquakes, seafloor slumping and volcano collapse can all cause tsunamis.”

And while an earthquake did occur in Papua New Guinea in 1998, it was the slumping of the seafloor — this indirect effect of the earthquake — that caused the devastating loss of life.  In 1999 Silver and colleagues combined sophisticated modeling with old-fashioned eyewitness testimony to identify the source of the tsunami.  A survey of the suspected stretch of seafloor with bathymetric imaging showed just what they anticipated:  a large slide.

“If you only tie tsunamis to earthquakes, you could be caught sorely unprepared,” said Silver.  “Seafloor slumping can occur at any time, so a tsunami could happen at any time.”

Something else that can happen at any time is the collapse of a volcano.  In 1888 most of Ritter Island, a neighbor island to Papua New Guinea, slid into the Pacific Ocean generating tsunami waves that may have reached 20 meters high (approximately six stories!). Silver returned to the waters off Papua New Guinea in 2004 aboard University of Hawaii research vessel Kilo Moana to map the seafloor adjacent to the islands in this volcanic arc.  The group, including Scripps’s Neal Driscoll and Bruce Appelgate, found evidence of 11 slides in addition to the Ritter collapse.

“This work has been vital in gaining a better understanding of the distribution and mechanics of submarine landslides around volcanoes, and how they create tsunamis. This is important because it enables better estimates of tsunami risks for coastal communities,” said Appelgate.

The bottom line is bleak for all those who want to live by the shore. “Every coast has some sort of continental slope offshore and those areas can slide so any coast could see a tsunami.”  Here’s hoping it happens only at their local movie theater.